The present invention relates to an internal combustion engine having electrohydraulic valve control for the variable-stroke driving of a gas exchange valve to which a spring force is applied in the closing direction, and a method for operating said internal combustion engine. This engine comprises a camshaft and a hydraulic system arranged to act as a drive between the camshaft and the gas exchange valve, said system being connected to a hydraulic fluid supply of the internal combustion engine and including the following:
a first hydraulic piston driven by a cam of the camshaft, and a second hydraulic piston that drives the gas exchange valve in the opening direction,
a pressure chamber delimited by the first hydraulic piston and by the second hydraulic piston, having a modifiable volume and a control channel that connects the pressure chamber to a pressure relief chamber,
an electrically controlled hydraulic valve, situated in the control channel, that permits a flow of hydraulic medium through the control channel in the open position of the hydraulic valve, and that blocks said flow in the closed position of the hydraulic valve.
The internal combustion engine further includes an electronic control module for controlling the hydraulic valve as a function of operating parameters of the internal combustion engine.
Internal combustion engines having electrohydraulic valve drives in which a partial volume of the pressure chamber, acting as a so-called hydraulic rod, can be continuously guided off into the pressure relief chamber when the hydraulic valve is open, so that the lift determined by the cam is correspondingly transmitted completely, partially, or not at all to the gas exchange valve, are known from many references in the patent literature. The design of the valve controlling of an internal combustion engine of this type is described in the article “Elektrohydraulische Ventilsteuerung mit dem ‘MultiAir’-Method [Electrohydraulic Valve Controlling with the ‘MultiAir’ Method],” published recently in the Motortechnische Zeitschrift (MTZ), December 2009. This article also indicates an engine characteristic map having different stroke curves that—based on the cam lift—are transmitted to the gas exchange valve in modified form as a function of the operating point by the subsequently connected hydraulic system. Also presented is the electronic control module for controlling the hydraulic valve, there in the form of an integrated engine control device.
Of course, the operating characteristic of electrohydraulic valve drives is significantly dependent on the properties of the hydraulic medium, and in particular its momentary state of viscosity, mainly influenced by temperature. An essential cause of this dependence is to be found in the so-called hydraulic valve brake, which is part of the hydraulic system and which replaces the valve closing ramp provided in conventional mechanical valve drives. As is known, the hydraulic valve brake has the task of forming a stroke of the gas exchange valve, decoupled from the cam lift, in such a way that the closing gas exchange valve always reaches the valve seat with a seating speed that is mechanically and acoustically acceptable. At the same time, the hydraulic valve brake is to be made such that the target/actual deviations of the gas exchange valve closing time point, which impair the charge changing of the internal combustion engine, are minimal.
Hydraulic valve brakes are customarily constructed such that shortly before the closing of the gas exchange valve the hydraulic medium displaced by the second hydraulic piston at the gas exchange valve side has to pass a throttle point whose hydraulic resistance produces a braking of the gas exchange valve stroke to the specified seating speed. However, the viscosity-temperature curve of the hydraulic medium, whose viscosity increases greatly as the temperature decreases, limits the functionality of the hydraulic valve brake to a temperature window in such a way that below a boundary temperature, the closing time point of the gas exchange valve fluctuates and/or is delayed in an impermissibly strong manner. In the extreme case, the gas exchange valve does not reach the valve seat at all, and, with regard to the charge changing and combustion processes of the internal combustion engine, remains open in an impermissible manner between two rotations of the camshaft.